1. Field of the Invention
The present invention relates to a method of making an optical fiber bundle, comprising individual optical fibers, which are compressed with each other at a common end in a common metallic sleeve, and shaped or formed by application of temperature and pressure. The invention also relates to an apparatus for performing this method. The invention further relates to an optical fiber bundle, which comprises a plurality of individual optical fibers, which are compressed and melted together at a common end in a hexagonal packing.
2. Prior Art
Frequently a flexible fiber optic light guide, comprising a plurality of individual optical fibers, a so-called optical fiber bundle, are used for light transmission. The individual fibers are usually combined at a common end in a sleeve, which, for example, is attached in a lighting apparatus for illumination.
The combination of the individual fibers at a common end to form a bundle requires special design engineering attention.
It is known in the prior art to make an optical fiber bundle by gluing the individual fibers together and bonding them in a sleeve that is pushed onto them. This widely used method has the disadvantage that the adhesive that is used limits the temperature resistance, the packing density and thus the optical transmission which is possible through the optical fiber bundle, because the individual optical fibers retain their circular cross-sectional shape and rest against each other only point-wise with free space. Also the chemical stability is limited which similarly reduces the range of applications in which this type of optical fiber bundle can be used.
Methods are known in which the individual fibers are melted with the sleeve and each other in a common sleeve.
The advantages of this sort of optical fiber bundle include above-all its higher temperature resistance (adhesive-free), higher light transmission, because more individual fibers are present in a cross-section by which hexagonal packing arises when the fibers are melted and compressed, and improved resistance to chemical attack, which is especially noteworthy for thermal disinfection applications and generally for sterilization in medical applications.
Processes are described in DE 26 30 730, in which a heat-softening sleeve is pressed on the optical fiber bundle. The individual fibers are then shaped hexagonally and there are no intervening free spaces between them. However in this prior art process melting between the individual optical fibers does not occur.
Glass sleeves would be advantageous to use because of their lower viscosity properties, but especially have the following disadvantages:
shaping tools cause imperfections and defects during sealing when used in glass sleeves;
the optical fibers are extremely impact- and shock-sensitive when used in glass sleeves (danger of chipping).
In the known method in principle metal sleeves with glass-like thermal and viscosity properties, i.e. the so-called heat-softening metals, could be used.
However these heat-softening metals have disadvantageous thermal and mechanical loads and are not usable in practice.
For protection of the glass sleeve the known method provides an outer metal sleeve surrounding the glass sleeve. The glass sleeve is pushed into a shaping or forming conical end of the outer metal sleeve and is compressed there by means of a slidable press-metal sleeve pushed on the fiber bundle. Then glass sleeve (and the press-metal sleeve) is connected with the outer metal sleeve by gluing or softened glass.
A formation of the end of the optical fiber bundle in this manner however has the following serious disadvantages:
The outer metal sleeve must be sufficiently thick-walled in order to compensate for thermal stresses (compression glass melt). This has the disadvantage that the usable optical surface area is small, in relation to the outer diameter of the metal sleeve.
An additional pressing tool that remains in the light guide ends is required to bring the inner glass sleeve into the outer metal sleeve. Problems result during centering of the optical fiber bundle in the softening glass sleeve, which act disadvantageously on the optical axis.
On inserting the inner glass sleeve into the outer metal sleeve furthermore no melting zone is formed, in which the individual fibers are parallel to each other. The conical convergence of the optical fiber bundle at their ends acts disadvantageously on the reflection properties of the optical fiber bundle.
only bundle diameters up to 10 mm can be made by this technique.
The provision of the outer metallic sleeve finally leads to an end portion of the optical fiber bundle that has three sleeves, namely
the glass sleeve (or alternatively a metallic sleeve made of a heat softening material),
a Press-sleeve,
the outer metallic sleeve, i.e. to a complex expensive termination of the optical fiber bundle as well is as the above-mentioned disadvantages.
There are additional disadvantages. In the known case a forming step, namely the compressing and tapering of the optical fiber bundle with heat and pressure, must be performed before putting on the glass sleeve (or alternatively the heat-softening metal sleeve).
DE 196 04 678 A1 describes a process in which the individual optical fibers are melted together at the end of the optical fiber bundle. Furthermore the entire optical fiber bundle (up to 30 m long) must be rotated to melt the common end, which leads to great manipulation difficulties and to limitations for more complex or larger components. In the known method the individual optical fibers and tools are placed in an electrically heated oven at the softening temperature. The melting process or softening process requires several hours for the case of large diameter components. A definite temperature adjustment of the temperature of the individual optical fibers to be melted is not possible because of the oven structure. Also only easily shaped materials (brass, nickel silver) with a very thin sleeve wall thickness can be used.
It is an object of the present invention to provide an improved method for making an optical fiber bundle based on the method according to German Patent Document DE 26 30 730 A1, so that an optical fiber bundle with improved optical properties and a wider range of possible applications results in a simple manner with simple means.
It is another object of the present invention to provide an improved optical fiber bundle having improved optical properties and a wider range of possible applications than the optical fiber bundles currently available in the art.
It is a further object of the present invention to provide an apparatus for making the optical fiber bundle according to the improved method for making it.
According to the invention the method of making an optical fiber bundle from a plurality of individual optical fibers that attains the above objects includes:
temporarily mechanically holding individual optical fibers together in a round and densely packed fiber bundle and pushing it in a snug fit in a single metallic sleeve made from a metallic material that has a sufficient high temperature strength at a forming temperature of the glass in the optical fibers;
installing the optical fiber bundle in a clamping device with the clamping device arranged in the vicinity of the single metallic sleeve in order to hold the optical fiber bundle fixed in an axial and radial direction;
heating the clamped end of the optical fiber bundle pushed in the single metallic sleeve to the forming temperature;
compressing the heated end of the optical fiber bundle in the single metallic sleeve to shape or form the individual optical fibers in a hexagonal packing and pressing in the metallic sleeve on this hexagonal packing, without sealing the single metallic sleeve to the optical fiber bundle;
cooling the shaped end of the optical fiber bundle with the individual optical fibers in the hexagonal packing; and
removing the optical fiber bundle from the clamping device.
The apparatus according to the invention for performing the above-described method for making the optical fiber bundle includes
a clamping device for stable holding of a single metallic sleeve made of metallic material with the optical fiber bundle mechanically held together and inserted completely in a cylindrical interior passage provided in the single metallic sleeve; and
an axially movable forming tool provided with an interior cavity tapered in a motion direction of the forming tool and shaped so that by moving the forming tool toward the end of said optical fiber bundle with the single metallic sleeve this end of the optical fiber bundle is compressed in a predetermined manner; and
an induction heater arranged in the vicinity of the forming tool so that the forming tool is heated by operation of the induction heater.
The invention also includes the optical fiber bundle made by the method according to the invention. This optical fiber bundle comprises a plurality of individual optical fibers melted together with each other at one end of the optical fiber bundle and a single metallic sleeve around the one end of the optical fiber bundle in which the individual optical fibers are inserted or pushed and compressed to form a hexagonal packing,
the single metallic sleeve is made of a metallic material with a high temperature strength sufficient for heating to a forming temperature for the optical fibers and with a cylindrical interior passage having a substantially circular cross-section, the single metallic sleeve compressed on the hexagonal packing is releasable in a nondestructive manner and the individual optical fibers in the hexagonal packing extend in an axial direction parallel to each other in the vicinity of the metallic sleeve.
The invention, in contrast to that disclosed in DE 26 30 730 A1, has no glass sleeve, but instead a metallic sleeve, which is made from a material that has a sufficient heat resistance at the forming or shaping temperature of the glass, i.e. a material which would not be usable in the prior art, since it would need to be a thermally softened metal.
In the case of the invention only a single sleeve made of heat-resistant metal is used, which has a through-going interior passage having a circular cross-section, i.e. it is a hollow cylinder and is exposed to only a single compression event or process, since the individual optical fibers are only held together by mechanical means and are directly or immediately pushed into the single high-temperature resistant metallic sleeve for compression, i.e. without preliminary shaping steps.
The following advantages result from the features of the invention:
optical fiber bundle with diameters up to 30 mm can be melted and provided with a terminal sleeve.
a larger optically active diameter in relation to the actual sleeve outer diameter is obtainable, i.e. the ratio of optically active bundle diameter to sleeve outer diameter isxe2x89xa70.8.
an outstanding transmission that is substantially larger than that obtainable with an optical fiber bundle in which the individual optical fibers are glued together is obtainable with an optical fiber bundle according to the invention having the same optically active diameter, because of the optimum hexagonal packing of the individual optical fibers over the entire cross-section of the optical fiber bundle according to the invention;
a most wide variety of sleeve materials are usable, especially corrosion-resistant materials, e.g. stainless steel for medical applications, and also nonferrous (NF) metals, such as brass and nickel silver.
no other parts are necessary for the melting step except for the metallic sleeve and the optical fiber bundle. To make the melted end of the optical fiber bundle no additional parts, such as an inner glass sleeve or pressing piston, are used, which are part of prior art techniques or are lost in the process, in addition to the metallic sleeve and the optical fiber bundle.
the length of the melted portion is freely selectable.
the heating for the melting/shaping of the light-conducting individual optical fibers can be performed so that a more rapid and reproducible forming or shaping process is guaranteed. Because of this aspect of the inventive process the shaping results in an entire optical fiber bundle of approximately hexagonally packed individual optical fibers and of very large diameter, up to 30 mm. This causes the fibers in the melted region to lie sufficiently parallel to each other and the melted region has very good centricity, which makes the propagation characteristics of the optical fiber bundle substantially better.
no strong bond between the outer metallic sleeve and the melted optical fiber bundle is produced by the shaping or forming process of the invention. Thus the optical fiber bundle can be made without the metallic sleeve simply by removing the metallic sleeve after the method according to the invention has been performed.
the regions of the optical fibers close to the sleeve wall are not damaged by the method.
According to a preferred embodiment of the method according to the invention a portion of the metallic sleeve that is exposed to the shaping process is provided with a coating, inside and outside, which serves as a separating layer between the shaped optical fiber bundle inside the sleeve and as a lubricating agent between the sleeve exterior wall and a compressing tool. This feature of the invention makes the shaping process with the appropriate tool easier and guarantees a nondestructive separation of the metallic sleeve. The coating occurs in a simple manner by dipping the metallic sleeve in a coating material, which is preferably formed by a suspension of boron nitride in ethanol. Of course other conventional coating methods may also be used as well as conventional high temperature-resistant coating materials, which can act as separating layer and lubricating layer, such as graphite, gold, etc.
A simple insertion of the optical fiber bundle into the metallic sleeve is attainable when the individual optical fibers forming the optical fiber bundle are held together temporarily by a bundling agent (e.g. an adhesive strip, string, wire, cable binder), so that the bundling agent can be easily manually removed or the bundling agent is stripped away automatically by the sleeve when the optical fiber bundle is inserted in the metallic sleeve.
A faster and sufficiently reproducible shaping process may be obtained, when the heating of the end of the optical fiber bundle inserted in the metallic sleeve is brought to a forming temperature by inductively generated heat, when a metallic heating element in contact with that end is inductively heated. The forming temperature is preferably in a range from 600 to 700xc2x0 C., a temperature at which the metallic material of the sleeve is still resistant to the heating, so that no strong bond is formed between the melted optical fibers and the outer metallic sleeve.
Besides the inductive heating other methods of producing the required forming temperature are also possible. However the inductive heating has a number of advantages. For example, in a preferred embodiment of the method the shaped end of the optical fiber bundle is cooled by gradual reduction of the input inductive power. Furthermore the inductive heating allows the heating element to provide an indirect heating of the optical fiber bundle at the same time as it is compressed by pressing it with this heating element which has a predetermined interior shape for this purpose. Because of that feature the same forming tool can be used to heat the end of the inserted optical fiber bundle to the forming temperature and for compressing its heated end.
According to another preferred embodiment of the invention the apparatus for performing the method is formed so that the interior cavity in the forming tool has a first cylindrical section at an open end thereof which has an inner diameter only slightly greater than the outer diameter of a cylindrical portion of the metallic sleeve receiving the optical fiber bundle prior to heating and compressing. The interior cavity in the forming tool includes a second cylindrical section at an end of the forming tool opposite from or remote from the open end and the second cylindrical section has an inner diameter corresponding to a predetermined desired diameter of the optical fiber bundle after the compressing. The interior cavity in the forming tool also includes a conical section connecting the first cylindrical section and the second cylindrical section. This particular specific structure for the forming tool permits the forming tool to be used for the various steps in the method for forming the optical fiber bundle according to its axial position for the end of the optical fiber bundle.
The interior surfaces of the second cylindrical and the conical sections of the forming tool are preferably hardened and polished because of the compressing step in the method.
In order to obtain the optimum hexagonal packing of the individual optical fibers over the entire cross-section of the light-guiding optical fiber bundle the above-described forming tool is structured so that the inner diameter of the second cylindrical section corresponds to the diameter of the optical fiber bundle less a distance corresponding to the hollow space between the optical fibers prior to heating and compressing, i.e. about 85% of the outer diameter of the metallic sleeve with conventional individual optical fibers having a diameter of about 30 xcexcm to 150 xcexcm.
In cases in which the terminal sleeve must have an especially small diameter, the diameter the second cylindrical section can be reduced still further so that a part of the fiber material, especially the fiber outer jacket, can be squeezed out of the sleeve. The exact extent depends on the nature and the structure of the individual fibers and must be determined from case to case. Generally the inner diameter can be reduced about 10% without difficulties.
In a preferred embodiment of the method of the invention temperature control of the forming or shaping temperature is provided so that a very good reproducible process control results, i.e. a sufficiently reproducible forming process. This is accomplished by providing a temperature controlling device for adjusting of the forming temperature in connection with and in the vicinity of the forming tool.
According to another preferred embodiment the metallic sleeve has a thick-walled section with a formed or molded element for a positive-locking engagement with a clamping device in order to guarantee that the metallic sleeve will reliably take the forming forces applied to it. The end of the light-guiding fiber optic bundle to be shaped is thus received in a thin-walled section of the metallic sleeve that does not impede the thermal shaping process because of its wall thickness. The term xe2x80x9cthick-walledxe2x80x9d means that the thick-walled section has a thicker wall than the corresponding thin-walled section. The term xe2x80x9cthin-walledxe2x80x9d means thin in relation to the thick-walled section.
According to another preferred feature of the invention the sleeve material is made from a corrosion-resistant stainless steel or a nickel-iron alloy. Also other metallic materials with suitably high-temperature resistance are usable, especially according to the range of applications foreseen for the optical fiber bundle.